1. Field of the Invention
The present invention relates to a flux applied in soldering to a substrate metal formed with a firmly oxidized layer such as stainless steel, chromate treatment steel or the like.
2. Description of the Prior Art
As a conventional flux of this kind for use in soldering, there has been proposed a mixed salt of phosphoric acid or zinc chloride-ammonium chloride. The mixed salt of phosphoric acid is gentle at a room temperature but greatly activated at a high temperature. Thus, the mixed salt of phosphoric acid acts as a flux useful for removing an oxidized layer from a substrate metal such as stainless steel at a high temperature. Similarly, the mixed salt of zinc chloride-ammonium chloride is strong in chemical activation and acts as a flux useful for removing the oxidized layer from the substrate metal. In this case, potassium chloride, sodium chloride or the like is added to the mixed salt to enhance stability of the flux at a high temperature.
In these conventional fluxes, however, the flux reaction of phosphoric acid occurs only at a limited temperature range, for instance, of 0.1.degree. C. at 213.degree. C. It is, therefore, very difficult to adjust the treatment temperature of phosphoric acid, particularly in an automatic soldering line. In use of the mixed salt of zinc chloride-ammonium chloride, a large amount of zinc ion is included in washing water of residue of the flux. It is, therefore, required to conduct higher order treatment of the washing water under a strict regulation of zinc ion concentration for discharge of the water.
Additionally, in use of the mixed salt of zinc chloride-ammonium chloride, the concentration of zinc metal at a crystal grain boundary and a joint interface in the soldering layer increases, resulting in occurrence of corrosion along the crystal grain boundary and part corrosion along the joint interface. The occurrence of such corrosion causes peeling off of the solder at the joint layer.